Resin transfer molding is a closed mold, low pressure molding process, sometimes referred to as liquid molding process, applicable to the fabrication of complex high performance composite articles of both large and small size. Several different resin transfer molding processes are well known to the skilled of the art. The process is differentiated from various other molding processes in that a reinforcement material, such as fiberglass or other fiber reinforcement material, is placed first into a molding tool cavity and then combined with resin within the mold cavity to form a fiber reinforced plastic ("FRP") composite product.
Typically, a pre-shaped fiber reinforcement, sometimes referred to as a reinforcement preform, is positioned within a molding tool cavity and the molding tool is then closed. A feed line connects the closed molding tool cavity with a supply of liquid resin and the resin is pumped or "transferred" into the tool cavity where it impregnates and envelops the fiber reinforcement and subsequently cures. The cured or semi-cured FRP product then is removed from the molding tool cavity. As used herein, the terms resin transfer molding and RTM are used to refer generically to molding processes wherein fiber reinforcement is positioned in a molding tool cavity into which resin is subsequently introduced. Thus, variations such as so-called press molding or squeeze molding, structural reaction injection molding ("SRIM") and the like are within the scope of such terms. Structural reaction injection molding uses a highly reactive resin system comprising two components pumped from separate holding tanks under pressure into an impingement mixing chamber and from there into the molding tool cavity. The tooling typically comprises a metallic shell to facilitate heat transfer. Although the mixing pressure is relatively high, the overall pressure of the resin in the molding tool typically is only about 50-100 psi. The resin flows into the molding tool cavity and wets-out the fiber reinforcement as the curing reaction is occurring. Typically, the fiber reinforcement material can be used in amounts up to about 20-30/weight percent of the fiber plus resin composite. Due to rapid resin cure, flow distances may be limited and for longer flow distances multiple inlet ports may be required.
Another variant of resin transfer molding, referred to generally as high speed resin transfer molding, is particularly suitable for commercial production of products requiring a three dimensional reinforcement preform. Fiber content typically is in the 35-50 weight percent range. Tooling for high production volumes typically is made of steel in order to contain molding pressures of 100-500 psi and for good heat transfer characteristics. For more limited production requirements, aluminum or zinc tooling may be acceptable. Typically, molding is carried out at elevated temperatures to reduce the cure time. The preform is positioned within the molding tool cavity, the mold is closed and resin is injected. At higher reinforcement levels, that is, at higher fiber weight content, the mold may be left slightly opened during resin injection to promote more rapid filling of the molding cavity; the mold cavity would then be fully closed. Preferably, the curing of the resin is accomplished in the mold such that the product will require no post-bake cycle and will have acceptable dimensional stability. For complex components or components having critical dimensional tolerance requirements, a fixtured post-cure may be required for adequate dimensional stability.
In view of the fact that RTM processes allow placement of fiber reinforcement materials, containing any of the various available fiber types or combinations thereof, in the mold cavity with minimal subsequent movement of the reinforcement preform during injection of the resin, the fiber reinforcement preform can be designed for optimum performance at minimum weight. That is, the fiber reinforcement preform can be designed and assembled with the most appropriate amount and type of reinforcement fiber (e.g., glass, graphite, aramid, etc., either chopped or continuous, random or oriented) in each portion of the preform. This yields a product of more optimum performance at reduced weight. Also, the low pressure required for resin injection often allows the use of less expensive presses and the use of tooling somewhat less costly than that employed in high pressure compression molding or thermoplastic stamping processes. Furthermore, there is the opportunity for significant assembly and tooling expense reduction where a significant degree of sub-part integration is achieved. That is, the RTM manufacture can integrate into a single, large, complex FRP component a number of sub-components which in metal would be manufactured separately and then assembled. In addition, the low pressures employed in RTM processes often enable larger structures to be produced than would be practical by other molding processes. Current compression molding processes, for example, are constrained by the cost and/or availability of sufficiently large presses.
Considerable effort is now being made to further advance the technology of RTM processes. Specifically, development is on-going in the areas of tooling fabrication, resin chemistry, control of resin flow and cure rates, and fabrication of complex preforms. With respect to fabrication of the preform, chopped, random fiber reinforcement material may be employed for its low cost and ease of use. One of the most versatile techniques for creating RTM-preforms, especially 3-dimensional preforms, is the so called spray-up process, wherein chopped glass roving or other chopped fiber reinforcement material is sprayed onto a forming mandrel from a chopper gun. Typically, the fibers are resin coated or a small amount of resin is introduced into the stream of chopped fibers to cause it to be retained on the screen. When the fibers accumulate to the proper weight or depth the resin can be cured to fix the shape of the resultant preform. Typically, the forming mandrel is a screen and vacuum is applied to the back of the screen to hold the fiber onto the screen as they accumulate and also to help ensure uniformity of fiber depth in the various areas of the screen. As the holes in the screen become covered by fiber, the remaining open areas tend to attract more fiber, causing a self-leveling action. This is capable of producing preforms of complex, near net shape with low waste.
A significant difficulty in the use of RTM processes, however, involves the fragile nature of the fiber reinforcement preforms. Preforms typically are handled and transported during manufacture and storage and during placement into the RTM molding tool cavity. Such handling and transport can cause damage, dislocation and loss of the reinforcement material of the preform. This can diminish the quality of the finished FRP product. Also, loose fibers can be a problem in the work area. In addition, when a preform is placed into a molding tool cavity, it must not extend beyond the desired seal or pinch off areas in the tool, since this could interfere with the mold closing and sealing properly. Particular care must be taken that the fibers of the reinforcement material do not extend from the preform into such areas or become dislodged and fall into such areas. This is a concern especially in the case of preforms, e.g. sprayed-up preforms as described above, in which chopped, randomly oriented fibers are employed. A covering is sometimes employed on a preform during shipment and handling, which covering is discarded prior to placement of the preform into the molding tool cavity. This fails to solve the problem, however, of reinforcement fibers being disrupted and lost during placement of the preform into the molding tool cavity. Thus, it fails to prevent loose fibers interfering with the closure and sealing of the molding tool cavity. It is an object of the present invention to provide an RTM preform comprising fiber reinforcement material (alone or with a foam or other core and with or without attachment fixtures and other features) wherein the reinforcement fibers are more durably held in position in the preform and better resist displacement and dislodgment. It is a further object to provide an RTM process employing such preform. It is one particular object of the invention to provide more durable preforms in which the fiber reinforcement material is less disrupted or dislodged by normal handling and transportation of the preform.